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First vertebrates went in for the kill

29 April 1995

By Douglas Palmer

MYSTERIOUS tooth-like fossils called conodonts that have baffled palaeontologists for more than 150 years are in fact the remains of the earliest vertebrates, according to researchers from Britain and South Africa. They have examined a rare fossilised example of one of the animals that bore the teeth, and have concluded that the creature was not only a vertebrate but also an active predator. The oldest vertebrates discovered previously were jawless fish that emerged 50 million years later than the conodonts.

Over the decades, conodonts have been shuffled from one animal group to another. The first conodont animal was discovered only in 1983, when an eel-like creature roughly 1 centimetre long was unearthed near Edinburgh. On the basis of this discovery, most palaeontologists agreed that the conodonts should be classified as primitive chordates. All chordates possess a stiffening rod called a notochord running the length of their bodies. The early chordates gave rise to vertebrates.

Conodonts first appeared some 520 million years ago. The claim that they were the first true vertebrates is based on the recent discovery of a “giant” conodont fossil, found near Cape Town in 440 million-year-old rocks. The creature would have been about 40 centimetres long, and the remains are so well preserved that even the soft tissues have been fossilised. Sarah Gabbott and Dick Aldridge of Leicester University and Hannes Theron of the South African Geological Survey in Bellville examined this soft tissue using a scanning electron microscope, and describe their findings in this week’s Nature (vol 374, p 800).

The fossil has V-shaped blocks of muscle containing tiny fibres 5 micrometres in diameter. These look just like those found in the muscles of lampreys and hagfish, jawless fish which are the most primitive surviving vertebrates. The animal’s eyes are particularly revealing. They carry strap-like muscles used for rotating the eye that are found only in vertebrates. The eyes’ position at the front of the head suggest that the conodonts were active predators.

This conclusion is also borne out by a new study of conodont teeth by Mark Purnell of Leicester University, who also publishes his findings in this week’s issue of Nature (vol 374, p 798). In the past, some palaeontologists disputed the idea that the familiar conodont fossils were teeth, saying that they did not show patterns of wear. Purnell has now found their surfaces do show microscopic signs of wear. The teeth were carried on tiny bars, producing comb-like structures that ran in pairs along the animals’ tubular mouths. The giant conodont possessed 19 tooth bars. Those nearest the front carried spiky teeth for grasping, while further back came blades for shearing and ridged plates for grinding. This arrangement only makes sense if the animals were predators. They “represent a major ecological shift from suspension feeding to actively hunting down prey”, says Purnell.